1. Field of the Invention
The present invention relates to nanocomposites, a method for synthesizing the nanocomposites, and an electrochemical capacitor comprising the nanocomposites. More specifically, the present invention relates to nanocomposites that have the advantages of easy process control, high electrical conductivity and large specific surface area, a method for synthesizing the nanocomposites, and an electrochemical capacitor comprising the nanocomposites.
2. Description of the Related Art
With the recent rapid development of portable electronic communication devices and hybrid vehicles, there has been an increasing demand for energy sources with high energy density and high power. In view of this demand, electrochemical capacitors have attracted attention as energy sources capable of replacing or supplementing conventional secondary batteries.
Electrochemical capacitors have lower energy density but retain much higher power density than existing secondary batteries. Therefore, electrochemical capacitors are advantageous in supplying high-power energy. In addition, electrochemical capacitors exhibit excellent performance characteristics in terms of charge/discharge time and cycle life over secondary batteries. Based on these advantages, electrochemical capacitors are suitable for use as pulse power sources of portable communication devices, energy sources for CMOS memory back-up, load-leveling systems for electric vehicles, and so forth. Recent research has also reported that the use of a hybrid energy source constructed by combining an existing secondary battery with an electrochemical capacitor contributed to a dramatic increase in the cycle life of the secondary battery.
Electrochemical capacitors can be largely classified into two different types: electrochemical double layer capacitors (EDLCs) and pseudo capacitors (or supercapacitors). An advantage of EDLCs is that high capacitance can be achieved by the use of an electrode active material having a large specific surface area. However, EDLCs are limited to low-current applications because of low actual storage capacity resulting from the presence of fine pores of activated charcoal as an electrode active material and incomplete wetting of electrolytes, and high internal resistance of the carbon system.
Pseudo capacitors use secondary oxidation/reduction reactions at the electrode/electrolyte interfaces and exhibit about 10-100 times higher capacitance than EDLCs. Electrode materials for pseudo capacitors include metal oxides, such as RuO2, IrO2, NiOx, CoOx and MnO2, and conductive polymers.
RuO2 has extremely high energy and power densities but is highly priced, which limits its use to aerospace engineering and military fields. The low capacitance of the other electrode materials leaves great room for improvement.
Thus, there is a need to produce high-capacity metal oxide electrodes at low cost. Recently, nickel oxide has received considerable attention as an electrode material for next-generation batteries and capacitors due to its large specific surface area and high theoretical capacity.
Metal oxides in the form of micrometer-sized powders have typically been used as electrode active materials for pseudo capacitors. Most reactions occur in a depth of several nanometers from the surface of metal oxides to accumulate and generate electricity. Accordingly, the use of metal oxide in the form of a micrometer-sized powder does not contribute to the improvement of capacity because electrochemical effects are not substantially attained within the metal oxide particles. Under these circumstances, considerable research efforts have been made in developing nanometer-sized metal oxides with maximal electrochemical utilization.
To find applications as electrode materials, metal oxides are required to have high electrical conductivity and porosity, and to be processed on a nanometer scale. To meet these requirements, attempts have been made to develop metal oxide/carbon composite materials that are configured to provide continuous conductive paths and have a three-dimensional porous structure while maintaining high electrical conductivity to maximize the impregnation with electrolytes and the interfaces with electrode active materials.
Electrochemical and chemical methods have been known as representative methods to coat metal oxides (e.g., nickel oxide) as active materials on carbon materials (e.g., carbon nanotubes) on a nanometer scale.
According to a typical electrochemical method, a carbon nanotube thin film is grown on a platinum-coated silicon wafer by electrostatic spray deposition (ESD) to produce a substrate, and then the substrate is immersed in an aqueous solution of nickel nitrate, followed by galvanostatic pulse deposition to coat nickel hydroxide on the substrate (Journal of the electrochemical society, 152, 11A2123, 2005). Despite the advantages that relatively uniform coating is possible and the coating thickness can be regulated by controlling the electric energy, the method is not practically used on an industrial scale because the active material is in the form of a thin film rather than in the form of a powder.
According to a typical chemical method, carbon nanotubes are dispersed in an aqueous solution of nickel nitrate and then an aqueous solution of potassium hydroxide or ammonium hydroxide is added thereto to precipitate nickel hydroxide (J. Electrochem. Soc., 153, A743, 2006; Synthetic Metals, 150, 153, 2005). However, the carbon nanotubes are simply added for the formation and growth reactions of homogeneous nickel hydroxide nuclei, causing problems in that uniform coating of the nickel hydroxide is difficult, the nickel hydroxide tends to aggregate and precipitate due to the formation of homogenous nuclei between the linear carbon nanotube strands, and the carbon nanotubes are embedded in an excess of the nickel hydroxide particles.